It is the date that if you were to purchase shares that had gone Ex Div you would not receive the Dividend payment. Consequently if you held the stock you could now sell it and retain the dividend.
The reason this can be an issue for the market is that the share price usually falls by the amount of the dividend due. Therefore if there a number of the larger constituents of the FTSE going Ex Div this can have an impact.
Hope this makes sense.
